Conventionally, switching devices for switching over super high frequency range signals in the GHz band typically included diodes serving as electronic switching devices. Such devices utilize the property of the diode to function as a switch depending on the way DC current is conducted through the diode. As other similar devices, there are switching devices which make use of slide switches, reed relays and other mechanical switching elements.
However, such switching devices using diodes create a substantial loss by their introduction into super high frequency electric circuits, and particularly when a plurality of such devices are connected in series the overall signal loss is so great that the use of such switching devices is not practical.
Also, slide switches are not suitable for switching over signals while maintaining impedance matching of an impedance line, and such a mismatching is typically so large even in frequencies in the order of 100 MHz that they are practically unusable in super high frequency ranges.
Reed relays can be used in super high frequency bands depending on their structure, but special considerations are required for connecting them with external circuits, and they tend to be large in size and expensive to manufacture.
To eliminate such problems of the prior art, the inventor proposed a new super high frequency switching device in Japanese patent application No. 63-234289.
According to this proposal, a ground electrode is formed on one side of a dielectric layer, and common fixed contacts are formed on the opposite side of the dielectric layer. The common fixed contacts are each interposed by a pair of selectable fixed contacts on either side, and a shield electrode is placed between each opposing pair of a selectable fixed contact and a common fixed contact. W-shaped moveable contact springs each having a pair of legs adapted to slide over an associated one of the common fixed contacts and one of the associated selectable contacts, respectively, are received in a spring holder which is slidably supported above the common fixed contacts and the selectable fixed contacts.
However, according to this structure, because the shield electrodes are located between the selectable fixed contacts and the common fixed contacts, the moveable contacts springs are grounded temporarily during their sliding movement. Therefore, if the moveable contact springs are connected to ECL or other external circuits including ICs, the ICs could be destroyed in some cases, and it is therefore necessary to take special measures such as effectively grounding the shield electrodes via capacitors for high frequency current.
Such a super high frequency switching device is typically used in a variable delay line for switching over super high frequency signals, and the frequency range of the delay line must cover a wide range extending from DC to super high frequency ranges. It is therefore necessary to set the resonant frequency as determined by the inductance of the shield electrodes and the capacitance of the capacitors higher than the frequency range of the delay line. Therefore, the circuit design is made highly complex, and the control of the resonant frequency requires special considerations because the capacitance of such capacitors can vary over a certain range. These factors increase the cost of the switching device.
It is conceivable to eliminate the shield electrodes. It is then necessary to reduce the spacing between the common fixed contacts and the selectable fixed contacts so that the moveable contacts can slide over them in a smooth manner without dropping into the gaps therebetween. When the spacing is reduced to a certain extent, a considerable electrostatic capacitance is formed therebetween, and will cause signal cross-talks.